A comprehensive hydro-thermo-chemo-mechanical model for predicting the early-age properties development of slag-blended cementitious systems is presented in this study. A new kinetic evolution for slag reaction that takes into account the heterogeneous nucleation effect and induction time of slag was proposed, with which the model effectively quantifies the contributions of slag to the hydration, energy exchange and mass conversation within blended systems. This paper provides a thorough documentation of input parameters, fundamental equations, and constitutive laws associated with the model. The hydro-thermo-chemo-mechanical model successfully predicts the heat release, hydration degrees of cement and slag, self-desiccation, basic creep and autogenous shrinkage in slag-blended cementitious systems. The predictions align well with the experimental observation obtained from this work and reported works in the literature, validating the accuracy and reliability of the proposed model in capturing and explaining the complex early-age behaviors of slag-blended cementitious systems.